Error 500 when uploading cookbook with knife (S3 bookshelf) - Error 400 bad request from S3

I am getting and error 500 when I try to upload a cookbook with knife. Other subcommands are successful and return valid data however. I suspect some issue with my custom bookshelf could be responsible.
To clarify my bookshelf is supposed to be a S3 bucket and my Postgres DB is in RDS.
I included these lines in the chef-server.rb:

postgresql['db_superuser'] = 'pivotal'
postgresql['db_superuser_password'] = '<some password>'
postgresql['external'] = true
postgresql['vip'] = '<my rds address>'
postgresql['port'] = 5394
bookshelf['access_key_id'] = '<my key>'
bookshelf['secret_access_key'] = '<my secret>'
opscode_erchef['s3_bucket'] = '<my bucket name>'
bookshelf['vip'] = 's3-eu-central-1.amazonaws.com'

Searching for a solution to my problem I have also tried variations of this with:

• bookshelf['enable'] = false
• passwords/keys entered via set-secret
• hardcoding my FQDN into the config and hosts file (it is a Route53 entry otherwise, which as I said works for most things)

None of these were able to fix the problem for me. I have tested accessing both the database and the bucket manually, but from the same machine and with the same credentials. Both work. The database is also filled after chef-server-ctl reconfigure, so I doubt having it external contributes to the problem.

I am able to create users and organizations, view information about them remotely via knife, etc. Only when I come to cookbooks do I get errors.

Start with the chef-server.rb settings for the external PostgreSQL database and S3 bookshelf, then trace the knife cookbook upload path and the reported S3 400 response. Compare it with the successful knife subcommands and verify the result by uploading a cookbook successfully.